"Another concern remained: The data was too clear, the pattern too obvious. "Surely, someone has thought of this before," Clauset recalls thinking. But the only example they could find of similar research was the work of Lewis Fry Richardson, an English mathematician and Quaker pacifist who drove an ambulance in World War I. Along with doing major work in the fields of weather forecasting and fractals, Richardson would go on to analyze all the wars from 1815 to 1945 and conclude that they followed a power-law distribution; he found the same pattern in gang killings in Chicago and Shanghai"

and

"At the prestigious Joint Statistical Meetings this August, however, Clauset and Woodard will return to the prediction issue: They will present new research concluding that if the level of terrorism violence remains stable — as it has, more or less, for decades — there's roughly a 30 percent chance of an attack similar to 9/11 in the next decade. To hedge their bets, they'll also note that if terrorism violence begins to ebb, that chance drops to just 7 or 8 percent. But if terrorism gets worse, the chances of another 9/11 increase significantly — to 80 percent."

Stefan Jovanovich writes:

Richardson's fundamental observation is one that anyone of sense has to agree with - "world" wars are disastrous beyond measure. The 2 World Wars– the only magnitude 7s in Richardson's classifications — were responsible for 3 out of every 5 wartime deaths in the period between the end of the Napoleonic War and VJ-Day. But those of us who cannot hack the math find one aspect of Richardson's study to be just plain bad history. By using logarithms he manages to fit a number of wars of fundamentally different importance into the same "magnitude" — a Richardson 6 war can have anywhere from 316,228 to 3,162,278 deaths.

One other random observation: those of us who love the 18th century above all others (Mozart, Washington, Watt, and Lavoisier) have this fact in our favor — the 1700s were the one period in modern (i.e. post 1400) history that had a complete absence of mass killing in war.